gluTessVertex describes a vertex
on a polygon that the program defines. Successive gluTessVertex calls describe
a closed contour. For example, to describe a quadrilateral gluTessVertex
should be called four times. gluTessVertex can only be called between gluTessBeginContour
and gluTessEndContour.

data normally points to a structure containing the
vertex location, as well as other per-vertex attributes such as color and
normal. This pointer is passed back to the user through the GLU_TESS_VERTEX
or GLU_TESS_VERTEX_DATA callback after tessellation (see the gluTessCallback
reference page).

It is a common error to use a local variable
for location or data and store values into it as part of a loop. For example:
for (i = 0; i < NVERTICES; ++i) { GLdouble data[3];
data[0] = vertex[i][0];
data[1] = vertex[i][1];
data[2] = vertex[i][2];
gluTessVertex(tobj, data, data);
}

This doesn't work. Because the pointers specified by location and data
might not be dereferenced until gluTessEndPolygon is executed, all the
vertex coordinates but the very last set could be overwritten before tessellation
begins.

Two common symptoms of this problem are consists of a single point
(when a local variable is used for data) and a GLU_TESS_NEED_COMBINE_CALLBACK
error (when a local variable is used for location).